Recently, with development of mobile communication service, high-quality service for non-service areas from secluded places among the mountains and remote islands which are not covered by mobile communication service to basements and apartment areas which are left as poor propagation areas in downtowns is required. However, to fulfill the requirement, it is necessary to build base stations in areas where telephone traffic is low at enormous expense, which is not profitable for mobile communication service companies due to inefficiency of investment and causes waste of national resources. Accordingly, a method for securing coverage and increasing communication quality in different concept than an existing mobile communication system is desired. In advanced nations, a repeater system using a CATV network is positively considered. When the geographical features of Korea having more than 80% of the land occupied by mountains and hills are considered, there is much limitation in using a microwave or a laser repeater system needing to secure the line-of-sight between a base station and a repeater station.
FIG. 1 is a schematic diagram of the configuration of a conventional optical communication system. The conventional optical communication system 100 includes a base station 110 and a repeater 120. The repeater 120 includes a master unit 125 and slave units 125a through 125d. Although only one master unit 125 is shown in FIG. 1, three master units 125 can be connected to the base station 110, and the four slave units 125a through 125d can be connected to each of the master units 125.
The base station 110 serves as a bridge connecting an exchange (not shown) to mobile terminals. The base station 110 is connected to the exchange through a cable network and connected to mobile terminals through a wireless network. The base station 110 includes an antenna for connecting these cable and wireless networks, a transmitter, a receiver, and a power supply unit and has a matching function for connecting the exchange to mobile terminals.
The master unit 125 of the repeater 120 is connected to the base station 110. The slave units 125a through 125d are linked to the master unit 125 through optical cables. Here, a wavelength of 1310 nm is used for optical transmission from the base station 110 to the master unit 125, and a wavelength of 1550 nm is used for optical transmission from the master unit 125 to the base station 110. Such optical communication system 100 transmits and receives electric waves through an optical cable, thereby achieving excellent quality of communication with minimum propagation loss. When it is considered that the service radius of the base station 110 is within about 1.5 km in the downtown area and is within about 5 km in the suburban area, the optical communication system 100 can provide a service over a wide area since a maximum of 12 slave units per base station 110 can be installed within the radius of 20 km.
Such optical communication system 100 necessarily requires a multiplexing apparatus for distributing a signal from a single point to many points or converging signals input from many points on a single point in a subscriber network such as CATV or Fiber In To Loop (FITL) developed with accomplishment of high-technology information communication network. Such a multiplexing apparatus is not that simple it is in an existing communication cable network. It can be realized as a special apparatus, i.e., an optical coupler. Optical couplers from a very simple 1H2 type which divides a signal received through a single channel into two signals to a complex 32H32 type which receives signals through 32 channels and distributes the signals through 32 channels have been commercialized. In the early stage of optical communication, instead of using optical couplers, optical fiber and optical cable channels are increased, thereby complicating communication networks. Use of optical couplers accomplishes simple communication networks. There are different types of couplers such as star couplers, directional couplers, and tree couplers according to the configuration of an information communication network.
Additionally, wavelength division multiplexer (WDM) is used as a special coupler and divide a wavelength of light. For example, a WDM divides a wavelength of 1310 nm from a wavelength of 1550 nm, inserts two different signals at the two different wavelengths, and transmits the two different signals through a single channel, i.e., an optical fiber cable. In addition, the WDM receives a mixed signal and divides it into two signals. Mechanisms of WDM include a grating method using the principle that refraction and reflection vary with wavelengths and a dichroic coating method using the principle that a particular wavelength is transmitted through a filter but all other wavelengths are reflected from the filter.
However, conventional optical communication systems have the following problems. First, since a network is established in a 1:1, 1:2, or 1:3 mode depending on installation and extensivity of master units and slave units, the supply of repeaters is in saturation, and additional optical cables must be installed to additionally establish slave units. In this case, additional installation of optical cables needs additional expenses, increasing financial burden on communication service companies. Second, since slave units are not controlled by a base station but are controlled by a master unit, communication between slave units is restricted. Communication between slave units can be accomplished, but it is needed to provide additional equipment for slave units. Third, in a multiplexing method using optical couplers, it is difficult to change wavelengths, and big and small loss according to a divergence ratio occurs during transmission. Since a fixed particular wavelength is used to distribute a desired wavelength or mix desired wavelengths using an optical coupler, it is difficult to adapt a conventional optical communication system to a flexible environment.